The Nittany Lions ended the 2013 season on their highest on-field note in years, upsetting Wisconsin in Camp Randall Stadium behind a career game from true freshman quarterback Christian Hackenberg, who threw for 339 yards, four touchdowns and zero interceptions. The 18-year-old passer looks like a player this team can build around.

The winds of change are swirling in Happy Valley, but now, at the least opportune time, a potential storm cloud has moved into the picture again.

According to CBS Sports' Jason La Canfora, NFL teams have been calling about head coach Bill O'Brien—the man most responsible for righting this ship. More importantly, O'Brien has been listening:

Penn State's Bill O'Brien, who came close to taking the Eagles job a year ago and was approached by several other teams, is ready to return to professional football, according to pro and college sources, and has already been approached by the Texans and Vikings.

O'Brien struggled with the decision last year and ultimately felt he owed it to his players to stay at Happy Valley for a second season, but relations between him and the school frayed some when the school was subjected to more post-Sandusky sanctions than expected, and he also has had three staff members depart recently.

One of those staff losses was a big one: linebackers coach Ron Vanderlinden, one of just two assistants O'Brien retained from the Joe Paterno era. Vanderlinden helped turn Penn State into "Linebacker U," coaching up standouts like Paul Posluszny, Dan Connor, Sean Lee and NaVorro Bowman in recent years.

Vanderlinden and quarterbacks coach Charlie Fisher both resigned of their own accord, leaving "to pursue other opportunities," according to Stephen Pianovich of the Philadelphia Daily News. That is never a good sign for a program, especially in conjunction with La Canfora's sources, which indicate that O'Brien might be willing to follow their lead.

Even if it's mostly smoke and mirrors, though, Penn State fans should be deeply worried about this story. O'Brien leaving would obviously set back the rebuilding effort at State College, but even a prolonged "will he, won't he" saga could have devastating effects on recruiting.

O'Brien has done an excellent job on the recruiting trail since taking over for Paterno in 2012—in general, sure, but especially considering the circumstances. Despite Penn State's postseason ban and scholarship restrictions, O'Brien's passion for and fealty to the school has endeared him to recruits, convincing blue-chip guys like Hackenberg to buy into the rebuilding process and commit.

But now that all might change.

High school players will not be so eager to believe O'Brien's schtick if his name is swirling in NFL rumors. And because of his NFL connections—O'Brien coached under Bill Belichick in New England from 2007 to 2011—those rumors will not be treated as mere speculation. He's thoroughly qualified to coach in the NFL, something every recruit in the country should know.

If he does leave, O'Brien's brief spell in Happy Valley would have to be considered a successful one. Penn State fans couldn't rightfully be outraged at him for leaving to coach in the pros—especially if part of his rationale is Sandusky-related.

O'Brien did nothing wrong to inherit that historic mess, and the job he has done in piecing things back together has been great. He has this program going in the right direction, but unless he can somehow convince fans and prospects that he doesn't plan on leaving, that momentum will soon be altered.

Penn State fans have experienced some bad days in the past couple of years; compared to the swarm of bees they've been subjected to, Sunday's report is like having a fruit fly land on their shoulder.